The present invention relates to techniques and mechanisms for calibration of optical analyzing devices or systems and more particularly to rare-earth doped optical mediums to calibrate UV absorbance detectors, methods for making such optical mediums and methods for calibrating devices using such optical calibration mediums.
Ultraviolet (UV) absorbance detectors or detection systems, such as monochromator based liquid/gas chromatographic detectors or spectrographs, typically are used in manufacturing facilities, hospitals and laboratories to analyze a sample to determine its chemical composition or make-up. The sample being analyzed can be an unknown material sample, e.g., a forensic analysis sample or a sample of a known material that is analyzed to verify its chemical composition, e.g. a sample of the raw material being used in a manufacturing process (e.g., pharmaceuticals). As such, these detectors or detection systems are calibrated by the manufacturer for delivery to the user and periodically thereafter to assure the detector/detection system is repeatedly and accurately sensing the spectral emissions representative of the material sample being analyzed. There are a number of techniques that can be used for field calibration of UV absorbance detectors. For purposes of the subject application, field calibration shall be understood to mean calibration of an instrument, detector or detection system at the end users location and not in a dedicated laboratory, manufacturing or testing facility, which generally is referred to as shop or lab testing.
One calibration technique involves the use of a light source, such as mercury pen-ray lamp, having a known spectral emission to calibrate the detector or detection system (i.e., calibration light source). Such calibration light sources provide for accuracy in wavelength calibration because of the generous range of their spectral features (e.g., emission peaks). For example, the range of spectral features for a mercury pen-ray lamp covers the region from 254 nm to 580 nm. Simply, a calibration light source has a number of well defined and known spectral peaks or valleys that can be easily and repeatedly identified by a detector.
Notwithstanding its advantages, this technique is inconvenient and time consuming particularly when used for field calibration. To calibrate a UV detector or detection system in the field, it is shutdown and then disassembled so the light source normally used for analysis (i.e., analysis light source) can be removed and the calibration light source installed in its place. In other words, the detector or detection system is re-configured with the calibration light source specifically for the purposes of its calibration.
After re-configurement is completed, the detector or detection system is turned on and the calibration light source is run for a sufficient period of time to stabilize the lamp""s spectral emissions. For example, it is typically recommended that a mercury pen-ray lamp be on for about 30 minutes to 1 hour before starting any calibration actions.
Thereafter, the detector or detection system is operated to determine the spectral emissions of the light source in relation to the detector""s/system""s performance or operation. For example, each position of a rotating diffraction grating of a monochromator detector or detection system is related to the wavelength of light reaching the UV sensor. In this way, the end user can correlate each position of the diffraction grating to a specific wavelength and the related bandpass of radiation that would irradiate a sample for analysis.
When the above actions are completed, the technician shuts the detector or detection system off, removes the calibration light source and re-installs the analysis light source. The technician then turns the detector back on and allows it to equilibrate to a stable operating condition.
The analysis light source, e.g., a deuterium lamp, typically has a characteristic spectral feature (e.g., see FIG. 5). After the unit has stabilized for purposes of spectral emissions, a quick test is typically run to see if the spectral characteristic of the analysis light source is seen where it is supposed to be. For example, the spectral emissions about and at a given rotational position of the rotating diffraction grating, corresponding to this wavelength characteristic, are evaluated to see if the position does corresponds to the wavelength of the analysis light source""s characteristic.
It is not uncommon to see a technician take about 2-3 hours, and more if there are adjustments or problems, to perform the above described calibration testing process. Because of the testing process and the need for a calibration light kit, it is also not uncommon to see this type of calibration test done by the manufacturer""s field representatives. As such, this technique does not allow xe2x80x9con-demandxe2x80x9d tests by the end user to be performed easily or without undue complexity.
In a second technique, a holmium doped glass filter is selectively disposed between the analysis light source and a UV sensor of the system. In one configuration, a holmium doped glass optical filter is disposed between the light source and the entrance slit for the detector. In another configuration, the holmium doped glass optical filter is disposed between the detector""s/system""s sample cell and UV sensor. The holmium glass filter in conjunction with the analysis light source generates an emission spectrum with distinct spectral features that can be used for wavelength calibration of a spectrophotometer and some HPLC detectors. In contrast to the first calibration technique described above, the holmium glass filter based calibration technique can be incorporated into the design and function of the instrument so the user can make an xe2x80x9con-demandxe2x80x9d type of test.
However, there are inherent shortcomings when using the holmium glass filter for UV instruments. Specifically, the holmium doped glass lacks far UV spectral features. Although holmium in a solution does exhibit spectral features in the range from about 240 nm to about 880 nm, as a practical matter holmium doped glass is only useful down to about 330 nm (e.g., see FIG. 6). Spectral features below 345 nm are difficult, if not impossible, to resolve because of the transmission cutoff of the base glass doped with the holmium material.
Conventional methods of doping optical glass requires the melting of the base glass, adding the required dopants and letting the glass cool and solidify. The solidified glass is then further processed (e.g., machined/polished) to obtain the finished part geometry. To overcome the poor UV transmission characteristic inherent in the base glass material described above, one could use a base glass such as quartz or fused silica. However, the extreme high temperatures required to melt quartz or fused silica, e.g. greater than about 1800xc2x0 C., restricts the selection of suitable dopants. In particular, these high temperatures essentially preclude doping base glass with a rare-earth material because the end product will not exhibit the desired spectral characteristic(s).
The absence of a useful spectral feature in the far UV range means that algorithms must be used to extrapolate the wavelength scale of the instrument over the spectral region between 190 nm and 345 nm. This is the spectral wavelength region in which the vast majority of UV absorbance detectors are operated in.
In a third technique, the detection system is initially calibrated using a calibration light source lamp at the manufacturer""s site or by a field service representative in the manner described above. The end user then periodically checks calibration by using spectral features inherent in the light source used for analysis. For example, in the case of deuterium lamp, one uses the 486 nm and 656 nm spectral lines (see FIG. 5). Although this method is convenient and accurate for the spectral region close to and between these lines, its accuracy and repeatability outside of these areas, particularly when dealing with wavelengths below about 350 nm, is suspect. This method or technique also relies on algorithms that extrapolate over a much larger spectral region than when using the holmium filter technique described above.
In another technique, a chemical standard is periodically analyzed by the detection system. For example, an erbium perchlorate liquid sample is analyzed and a spectral emission or characteristic as a function of the operation of the detection system is obtained. The end user uses the obtained spectral characteristic of the controlled sample to calibrate the instrument, detector or detection system. This test is similar to the validation tests done periodically to independently establish the spectral performance and/or the operability of the detection system.
In a validation test, the spectral emissions as determined by a calibrated detector/system is compared with the known spectral emission characteristic for the sample. If the comparison matches within a given degree of accuracy, then the detector""s or detection system""s results are considered to be validated and reliably accurate. However, if the comparison does not match within the required degree of accuracy, then the detector/system must be re-calibrated, repaired or replaced. Additionally, any test or analysis previously performed by the detector or detection system may be suspect.
Although chemical standards tests are accurate and can be used to calibrate as well as to validate a detector""s performance, they are time consuming and expensive, particularly if performed on an xe2x80x9con-demandxe2x80x9d basis, e.g., day to day calibration checks.
There also is described in U.S. Pat. Nos. 4, 099,883, 4,106,857 bandpass filters that include rare-earth material constituents. For these bandpass filters the spectral characteristics of combinations of various rare-earth materials are used to establish or define the cut-off or boundary for a given bandpass of non-UV wavelengths. There also is described in U.S. Pat. Nos. 5,311,525, 5,452,124, 5,467,218, 4,481,399, 5,502,592, 5,491,581, 5,067,789, 5,524,118, 5,474,588 and U.S. Pat. No. 5,526,459 a number of applications where a fiber, used for laser light communications, is doped with a rare-earth material.
The present invention features an optical medium uniquely adapted and configured for insitu calibration of UV absorbance detectors by end users or manufacturers service representatives. The invention also features methods for making such an optical medium as well as methods for calibrating UV absorbance detectors using the optical calibration medium of the invention. Additionally, the invention features a UV absorbance detector or system uniquely configured to facilitate performance of such a calibration by the user without disassembly.
The optical medium for calibration of UV absorbance type detectors, according to the present invention, includes a gel-silica base glass monolith and a rare-earth material dopant therein. In a particular embodiment, the gel-silica base glass monolith includes a type IV porous gel-silica base glass and more particularly includes a type V dense gel-silica base glass. The type IV doped gel-silica base glass has a UV transmittance of about 50% at 250 nm as compared to about a transmittance of about 3-4 for the base glass used for prior art holmium doped glass filters. Further, the characteristics of the rare-earth dopant in the far UV remain generally discernable.
In a particular embodiment, the dopant is selected from the group consisting of atoms of the rare-earth group that have partially filled 4f electron shells (namely from cerium, atomic number 58, to ytterbium, atomic number 70). More particularly, the rare-earth materials selected for use as dopants are those exhibiting a wide range of spectral features, preferably over a range from about 190 nm to about 700 nm and more particularly, from about 220 nm to about 700 nm. Preferably, the rare-earth dopants have at least one distinct spectral feature in the far UV, more particularly, in a range from about 190 nm to about 300 nm. More preferably, the rare-earth dopant is erbium, atomic number 68, having spectral features in a range from about 190 nm to about 650 nm and a distinguishable spectral feature at about 257 nm.
The above described optical medium for calibration of UV absorbance type detectors is made by a process including steps of mixing a slurry (sol) including silica, casting the sol into a rough final desired shape, allowing or causing the sol to solidify to produce a gel, aging the gel, drying the gel to remove the liquid phase and densifying the dried gel. The process further includes the step of doping at least one of the slurry or the gel with a rare-earth dopant. More particularly, doping the slurry or gel with erbium, in an exemplary embodiment, erbium nitrate. In a particular embodiment, the step of mixing includes adding the rare-earth dopant to the slurry being mixed. In another embodiment, the process further includes the step of impregnating the dried gel with the rare-earth dopant.
Preferably, the steps of aging, drying and densifying are performed under conditions that yield at least a type IV (porous) gel-silica base glass monolith having good far UV transmission characteristics. More particularly, the highest temperature used during these steps of aging, drying and densifying is about 900xc2x0 C. or less.
In a more particular embodiment, the optical medium of the invention is made by a process including steps of hydrolyzing and polycondensing one or more oxide precursors to form a sol including a plurality of oxide particles suspended in a liquid; casting the sol into a mold, gelling the sol by cross-linking oxide particles to form a gel; aging the gel to form an aged gel; subjecting the aged gel to a drying treatment to remove liquid from pores of the aged gel to form a dried gel; and densifying the dried gel to form an oxide sol-gel monolith. The drying treatment includes steps of (i) heating the aged gel in a mid to high humidity environment and then (ii) heating the aged gel in a low humidity environment. The foregoing steps also being Bet forth in U.S. Pat. No. 5,076,980, the teachings of which are incorporated herein by reference.
The above described process further includes the step of doping at least one of the sol or the gel with a rare-earth dopant. More particularly, doping the sol or gel with erbium, in an exemplary embodiment, erbium nitrate. In a particular embodiment, the step of hydrolyzing and polycondensing includes adding the rare-earth dopant, e.g., erbium to form the sol. In another embodiment the process further includes the step of impregnating the dried gel with the rare-earth dopant.
Preferably, the steps of aging, subjecting the aged gel to a drying treatment and densifying are performed under conditions that yield at least a type IV (porous) gel-silica base glass monolith. More particularly, the highest temperature used during these steps of aging, drying and densifying is less than about 900xc2x0 C.
As indicated above, the invention also features a method for calibrating any one of a number of UV absorbance detectors having a light source with spectral emissions over a range of wave-lengths and a sensor being responsive to at least a portion of the spectral emissions from the light source. The calibration method of the instant invention includes steps of providing a rare-earth doped gel-silica base glass monolith, selectively disposing the rare-earth doped gel-silica base glass monolith between the light source and the sensor, sensing the radiation passing through the rare-earth doped gel-silica base glass monolith, identifying spectral features unique to the light source and the rare-earth doped monolith and establishing a relationship between operation of the UV absorbance detector and a wavelength to be sensed using the identified spectral features.
In one specific embodiment, the rare-earth doped gel-silica base glass monolith is selectively disposed between the light source and an entrance slit of the UV absorbance detector. For this embodiment, the detector senses transmission. In another specific embodiment, the rare-earth doped gel-silica base glass monolith is selectively disposed between the sensor and the sample cell of the UV absorbance detector. For this embodiment, the detector senses absorbance.
For monochromator type UV absorbance detectors, the detector further includes a mechanism that selectively isolates a specific wavelength bandpass from the range of wavelengths being emitted by the light source. Additionally, the above described calibration process further includes the step of actuating the mechanism in stepwise fashion to sequentially isolate a bandpass over the range of wavelengths.
For spectrographic type UV absorbance detectors, the detector further includes a diffraction grating between the light source and the sensor, the sensor is arranged to receive the spread spectrum radiation from the diffraction grating and the sensor is configured to separately detect radiation in a plurality of bandpasses. Further, the step of selectively disposing the rare-earth doped gel-silica base glass monolith includes selectively disposing the monolith between the light source and an entrance slit of the UV absorbance detector so radiation passing through the monolith impinges on the diffraction grating. Additionally, the step of sensing includes simultaneously and separately sensing in a plurality of bandpasses the spread spectrum radiation from the diffraction grating.
In a particular embodiment, the rare-earth doped gel-silica base glass monolith being provided is an erbium doped gel-silica base glass monolith. Further, the step of sensing includes sensing spectral emissions in the far UV and the step of establishing relationship between operation and wavelength includes establishing a relationship with the wavelengths in the far UV based on the spectral emissions being sensed in the far UV.
As noted above, also featured is a UV absorbance detector or detection system incorporating a rare-earth doped gel-silica base glass monolith of the instant invention. The features of such a detector or system are hereinabove described, and as such, will not be repeated again here. In a preferred embodiment, such a detector or system further includes a mechanism for selectively interposing the rare-earth doped gel-silica base glass monolith between the system""s light source and the sensor. In this way, an end user can easily perform a calibration activity without having to turn the detector or system off as is done with the prior art calibration lamp technique. Such a mechanism is remotely operated either manually or by a remote actuator, for example a rotatory air-operated or electrically operated actuator.
Features of the instant invention include an optical medium that, in conjunction with the analysis light source of a UV absorbance detectors, generates a useful emission spectrum. This emission spectrum is useable to perform xe2x80x9con-demandxe2x80x9d calibrations of such UV absorbance detectors. In particular, the emission spectrum being generated covers a wide range of wavelengths, for example over the range from about 190 nm to about 700 nm. More particularly the spectrum being generated includes spectral features in the far UV range or in the range from about 190 nm to about 300 nm. Other features of the invention include a method of making a rare-earth doped optical medium that exhibits the above described characteristics and a method for calibrating such UV absorbance detectors using the a rare-earth doped optical medium of the invention.